AUGUST 2004 KLIMOWSKI ET AL. 727 Radar Observations of the Early Evolution of Bow Echoes BRIAN A. KLIMOWSKI National Weather Service, Flagstaff, Arizona MARK R. HJELMFELT South Dakota School of Mines & Technology, Rapid City, South Dakota MATTHEW J. BUNKERS National Weather Service, Rapid City, South Dakota (Manuscript received 10 April 2003, in ®nal form 11 February 2004) ABSTRACT The evolution of 273 bow echoes that occurred over the United States from 1996 to 2002 was examined, especially with regard to the radar re¯ectivity characteristics during the prebowing stage. It was found that bow echoes develop from the following three primary initial modes: (i) weakly organized (initially noninteracting) cells, (ii) squall lines, and (iii) supercells. Forty-®ve percent of the observed bow echoes evolved from weakly organized cells, 40% from squall lines, while 15% of the bow echoes were observed to evolve from supercells. Thunderstorm mergers were associated with the formation of bow echoes 50%±55% of the time, with the development of the bow echo proceeding quite rapidly after the merger in these cases. Similarly, it was found that bow echoes formed near, and moved generally along, synoptic-scale or mesoscale boundaries in about half of the cases (where data were available). The observed bow-echo evolutions demonstrated considerable regional variability, with squall line-to-bow- echo transitions most frequent over the eastern United States. Conversely, bow echoes typically developed from a group of weakly organized storms over the central United States. Bow-echo life spans were also longest, on average, over the southern plains; however, the modal life span was longest over the eastern United States. Finally, the supercell-to-bow-echo evolution was most common across the northern plains, but the data sample is too small for this result to be considered signi®cant. 1. Introduction ers roughly 40%±50% of the time, which in turn led to 1 bow-echo con®gurations and lifetimes of widely vary- Klimowski et al. (2003) found that bow echoes ac- ing spatial and temporal scales. Because the radar ap- count for nearly one-third of the convectively generated pearance of a bow echo is an indicator of severe winds severe wind reports over the northern High Plains of (not a predictor), it is important to understand the early the United States. Furthermore, 86% of the bow echoes evolution of bow echoes to better anticipate the pro- that they studied produced severe winds, with severe duction of the downbursts responsible for these bowing wind reports being 3 times as frequent as severe hail storms. Speci®cally, it is the purpose of this paper to reports. Klimowski et al. (2000, 2003) also observed identify (i) the convective structures preceding bow- that bow echoes were preceded by thunderstorm merg- echo development, and (ii) the varying morphologies and lifetimes of the resulting bow echoes. It should be 1 A bow echo is de®ned as a nontransient bow- or crescent-shaped noted that the dataset in the present study has been radar signature with a tight re¯ectivity gradient on the convex (lead- signi®cantly expanded upon from Klimowski et al. ing) edge, the evolution and horizontal structure of which is consistent (2003) to include cases over the majority of the United with storms that propagate along a strong out¯ow. Note that this de®nition is not speci®c in the size, duration, or evolution of the States (primarily regions east of the Rocky Mountains). storm, only its shape and association with strong winds (also see Fujita (1978, 1979) was the ®rst to de®ne the typical Fujita 1978; Przybylinski 1995; Weisman 2001; and Klimowski et evolutionary characteristics of the bow echo. According al. 2003). to Fujita (1979), the bow echo commonly evolves from either a single convective cell or a line of cells into a Corresponding author address: Dr. Brian A. Klimowski, National comma-shaped echo with a dominant cyclonic vortex Weather Service, P.O. Box 16057, Bellemont, AZ 86015-6057. on its poleward side. Weisman (1993, 2001) further il- E-mail: [email protected] lustrated the importance of bookend vortices and the q 2004 American Meteorological Society 728 WEATHER AND FORECASTING VOLUME 19 rear-in¯ow jet (RIJ) in sustaining and/or strengthening a bow echo. Bow echoes have been observed to occur over a wide range of sizes (e.g., 10±150 km), while evolving from a diverse set of morphologies (e.g., Przy- bylinski and DeCaire 1985; Johns and Hirt 1987; Moller et al. 1990; Lee et al. 1992). Even though bow echoes have been studied exten- sively through observational studies (Przybylinski and Gery 1983; Burgess and Smull 1990; Smith 1990; Przy- bylinski 1995; Funk et al. 1999, just to name a few) and numerical simulations (Weisman 1993, 2002; Finley et al. 2001), how they evolve from various initial con- vective structures has not been comprehensively doc- umented. For example, there are a few studies illus- trating that storm mergers sometimes occur prior to FIG. 1. The regional divisions used in the categorization of the bow-echo formation (e.g., Przybylinski and DeCaire bow echoes. The total number of bow echoes initiated within each 1985; Smith 1990; Finley et al. 2001), but they do not region is given in parentheses. provide any information on how frequently this occurs. Other studies have documented supercell-to-bow-echo 2. Nature of the dataset transitions (e.g., Moller et al. 1990; Conway et al. 1996; Klimowski et al. 1998), but, again, the frequency of this The identi®cation of bow echoes was performed using type of evolution is not well known. Rear-in¯ow notches radar re¯ectivity and velocity data derived from a va- (RINs), a manifestation of the RIJ, may also be present riety of Weather Surveillance Radar-1988 Doppler (on radar) before noticeable bowing occurs (e.g., Przy- (WSR-88D) sources.2 WSR-88D archive level II data bylinski and Gery 1983; Burgess and Smull 1990; Przy- were used for the majority of the cases from 1996 bylinski 1995), but this signature may not provide much through 1999. Where needed, these data were supple- warning before severe surface winds occur; it may only mented with the National Weather Service's Next Gen- be an indicator that severe winds are imminent or oc- eration Weather Radar (NEXRAD) Information Dissem- curring. Finally, most of these bow-echo studies involve ination Service (NIDS) data. With the availability of the individual cases, which prevents broader conclusions higher-resolution NIDS data after 1999, the majority of from being drawn. the radar data for the cases from 2000 to 2002 were Some of the more comprehensive bow-echo studies acquired from single-site and mosaic NIDS data. For have shed some light on their early evolution. Przy- all cases, the temporal (spatial) resolution of the radar bylinski and Gery (1983) examined radar re¯ectivity data were at least 10 min (1±2 km). structures from over 30 bow echoes. They found that Data for 65 bow-echo cases came from a Cooperative strong low-level re¯ectivity gradients and maximum Program for Operational Meteorology, Education, and storm tops often occurred near the leading edge of the Training (COMET) research project on convective high bow echo in the early stages of development; however, wind events over the northern High Plains from 1996 the radar characteristics leading up to this stage were to 1999 (Klimowski et al. 2003). In order to acquire as not presented. More recently, Schmocker et al. (1996) many bow-echo cases as possible, in addition to the used the midaltitude radial velocity convergence Klimowski et al. (2003) dataset, several methods of (MARC) signature to identify thunderstorms that would identi®cation were employed. Most directly, regional likely produce downbursts and eventual bow echoes. and national radar mosaics were perused daily to vi- Although Schmocker et al. focused on the early stages sually identify potential bow echoes. In the event that of bow-echo development, it is not clearly understood the review of available radar data were not possible, what convective structures typically precede these archived severe wind reports were scrutinized for pat- MARC signatures. Nevertheless, the MARC signature terns that might infer an organized high wind event has proven useful as warning guidance for the onset of (three or more severe wind reports grouped together). severe surface winds (e.g., Funk et al. 1998). In yet Radar data associated with the high wind reports were another regional study, Przybylinski et al. (2000) found then acquired and analyzed for potential bow echoes. that initial bow echoes frequently developed near the Analyses of radar data in this way (primarily from the intersection of a convective line and external boundary. years 2000 to 2002 and some earlier data) allowed for The overarching goal of all of these studies has been to provide better warning guidance for the early stages of these windstorms. Therefore, in the present paper, we 2 The identi®cation of a bow echo was subjectively based on the de®nition of bow echo given in the introduction. The initiation of are especially interested in the chain of events that ini- the bow echo was determined from the ®rst radar scan in which tiate the downbursts that eventually produce the bow- incipient bowing was present. The demise of the bow echo was that echo structure. time at which the de®nition of a bow echo could no longer be satis®ed. AUGUST 2004 KLIMOWSKI ET AL. 729 FIG. 2. The four general types of bow echoes described in the text: (a) BE, (b) BEC, (c) CBE, and (d) SLBE. The BE, BEC, and CBE are all relative to the scale as given in (b).